Nitrate δ15N and δ18O Values Reveal Mariculture Impacts on Nitrogen Cycling in Sansha Bay, SE China

Global fisheries’ production has intensified rapidly over recent decades, making significant contributions to food security and economic development. However, this growth has exerted pressure on marine ecosystems and altered coastal nitrogen cycling. This study focused on Sansha Bay, an important aquaculture harbor in SE China, with measurements of environmental parameters and nitrate N–O isotopic compositions, to explore the impact of distinct mariculture types on nitrogen cycling in a coastal farming bay. The mean nitrate concentration was 120.7 ± 24.2 μmol dm−3, with a mean N/P ratio of 69.6 ± 19.6, indicating that the water column is in a state of eutrophication. Different aquaculture practices influence the nutrient status of water, with mixed farming (shellfish, seaweed, and fish) increasing nitrate concentrations and reducing N/P ratios, while seaweed farming reduces nitrate concentrations and increases N/P ratios. In spring, the average δ15N and δ18O values of nitrate in the water column were 8.4‰ and 8.7‰, respectively, compared with values of 9.8‰ and 4‰ recorded previously in winter. Such variation may be attributed to the greater influence of seaweed farming in spring when nitrate cycling is driven mainly by assimilation. In terms of aquaculture type, nitrate assimilation was stronger in seaweed farming zones, and nitrification was more pronounced in mixed farming zones. Overall, the results demonstrate that different mariculture types in Sansha Bay influence nitrogen cycling in the water body, with seaweed-based aquaculture systems contributing to the sustainable development of marine aquaculture by effectively mitigating eutrophication.